Brightness enhancement film with improved view angle

ABSTRACT

A structural shape on the surface of an optical substrate is such that the brightness of diffuse light departing from the surface of the optical substrate at certain off axis angles is reduced, at the expense of a small reduction in peak brightness measured near the viewing axis. The net result is an overall increase in useful illumination. A cross section of a prism as the structural shape has a curved sidewall or facet. A material with a relatively high index of refraction combined with a prismatic structure having a modified prism geometry improves brightness.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U. S. application Ser. No.10/065,981, filed on Dec. 6, 2002.

BACKGROUND OF THE INVENTION

This invention relates to brightness enhancement films and, morespecifically to such films having curved facet prism structures andincreased prism peak angles and refractive indices.

In backlight computer displays or other systems, optical films arecommonly used to direct light. For example, in backlight displays,brightness enhancement films use prismatic structures to direct lightalong the viewing axis (i.e., an axis normal to the display, or “onaxis”). This enhances the brightness of the light viewed by the user ofthe display and allows the system to use less power to create a desiredlevel of on-axis illumination. Films for turning light can also be usedin a wide range of other optical designs, such as for projectiondisplays, traffic signals, and illuminated signs.

Backlight displays and other systems use layers of films stacked andarranged so that the prismatic surfaces thereof are perpendicular to oneanother and are sandwiched between other optical films known asdiffusers. Diffusers have highly irregular surfaces.

The use of current commercial brightness enhancement films causes asharp cut-off in brightness between about 40 and 50 degrees off-axis. Atangles beyond this cut-off there are side-lobes in the angularbrightness distribution. These side-lobes can result in a waste ofenergy because they are outside the desired viewing angle specificationsof many liquid crystal display (LCD) devices. The side-lobes are alsoundesirable in security applications since they allow light to reachunintended viewers.

Thus, there is a continuing and demonstrated need in the prior art forbrightness enhancement films which suppress sidelobes in the angulardistribution of brightness.

SUMMARY OF THE INVENTION

In a first embodiment, the invention features a structural shape for thesurface of an optical substrate such that the brightness of diffuselight departing from the surface of the optical substrate at certain offaxis angles is reduced, at the expense of a small reduction in peakbrightness measured near the viewing axis. The net result is an overallincrease in useful illumination. Such an optical substrate comprises asurface characterized by a cross section of at least one prism having acurved sidewall or facet.

In a second embodiment, the invention features a combination of a highindex of refraction prismatic structure with a modified prism geometry.Brightness performance is met or exceeded, for example in an LCD backlight display device, when the index of refraction of the prismstructure is increased to a value above the index of refraction ofmaterials commonly used in brightness enhancement films, while the peakangle is allowed to increase beyond 90 degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a backlight display device.

FIG. 2 is a perspective view of an optical substrate comprising asurface characterized by a cross section of a prism having a curvedsidewall or facet.

FIG. 3 is a first cross sectional view of an optical substratecomprising a surface characterized by a cross section of a prism havinga curved sidewall or facet.

FIG. 4 is a second cross sectional view of an optical substratecomprising a surface characterized by a cross section of a prism havinga curved sidewall or facet.

FIG. 5 is a graphical depiction of brightness as a function ofhorizontal viewing angle for an optical substrate comprising a surfacecharacterized first by a cross section of a right angle prism, second bythe curved sidewall or facet in FIG. 3 and third by the curved sidewallor facet in FIG. 4.

FIG. 6 is a cross sectional view of a compound angle prism and of thegeometric parameters of the curved sidewall or facet of FIGS. 3 and 4 asdescribed by a segment of a polynomial function.

FIG. 7 is a perspective view of two optical substrates positioned in acrossed configuration wherein prismatic structures are positioned at anangle with respect to one another (e.g., 90 degrees).

FIG. 8 is a map of the central luminance of crossed optical substratesas a function of the prism peak angle and the refractive index of thesubstrate.

FIG. 9 is a graphical depiction of the far field horizontal luminance ofcrossed optical substrates as a function of horizontal viewing angle.

FIG. 10 is a graphical depiction of the far field vertical luminance ofcrossed optical substrates as a function of vertical viewing angle.

FIG. 11 is a graphical depiction of the far field horizontal luminanceof crossed optical substrates as a function of horizontal viewing angle.

FIG. 12 is a graphical depiction of the far field vertical luminance ofcrossed optical substrates as a function of vertical viewing angle.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 a cross sectional view of a backlight display device 100 isshown. The backlight display device 100 comprises an optical source 102for generating light 104. A light guide 106 guides the light 104therealong by total internal reflection (TIR). The light guide 106contains disruptive features that cause the light 104 to escape thelight guide 106. A reflective substrate 108 positioned along the lowersurface of the light guide 106 reflects any light 104 escaping from thelower surface of the light guide 106 back through the light guide 106and toward an optical substrate 110. At least one optical substrate 110is receptive of the light 104 from the light guide 106. The opticalsubstrates 110 comprise a three-dimensional surface 112 defined byprismatic structures 116 (FIGS. 2, 3 and 4).

The optical substrates 110 may be positioned, one above the other, in acrossed configuration wherein the prismatic structures 116 arepositioned at an angle with respect to one another (e.g., 90 degrees) asseen in FIG. 7. The prisms 116 have a prescribed peak angle, a, aheight, h, a length, 1, and a pitch, p and one or both of the prismaticsurfaces 112 may be randomized in their peak angle, a, height, h,length, 1, and pitch, p. Yet further, one or both sides of thesubstrates 110 may have the prisms 116. In FIGS. 2, 3 and 4, in a firstembodiment of the invention, the sidewall or facets 132 of the prisms116 which comprise the surface 112 are curved. The curvature can bedescribed as a segment of a parabola, or more generally as a polynomialsurface given by the sag equation: $\begin{matrix}{z = {\frac{{cr}^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}r^{2}}}} + {dr}^{2} + {er}^{4} + {fr}^{6} + {{Higher}\quad{order}\quad{terms}\quad{in}\quad r}}} & (1)\end{matrix}$where z is the perpendicular deviation (or “sag”) in microns of thesidewall or facet 132 of the prisms 116 from a straight reference line128, originating at a first reference point (b) at a base of the prismand terminating at a second reference point (a) near the peak of theprism (see FIG. 6) and c⁻¹ is the radius of curvature of the facet. Herethe coefficients of the polynomial may have the following approximateranges: −20<c<20, −10<d<10, −10<e<10, −10<f<10, and −1<k or less than orequal to zero, wherein r is a radial coordinate or distance from anoptical axis in microns. It is noted that c²r² is greater than or equalto zero and less than or equal to 1. Odd order terms in r (e.g., r¹, r³,r⁵, r⁷, etc.) with appropriately chosen coefficients may also be used asin Eq. 1. The higher order terms for the even and odd order terms haveappropriately chosen coefficients. Terms other than the first r² termmay be written as: $\sum\limits_{i = 1}^{N}{a_{i}{r^{i}.}}$

Linear segments 124, 126 or other approximations to the polynomialdescribed by Eq. 1 may also be used as seen in FIG. 6. Linear segments124, 126 result in a compound angle prism having a first facet 126 at anangle of θ and a second facet 124 at an angle of β. As best understoodfrom FIG. 6, the curvature of the curved sidewall or facet 132 of theprisms 116 can be either convex or concave. In FIG. 6, the side facetsof the prism are positioned so as to form one or more compound facets124, 126, respectively subtending an angle of β or θ with the base ofthe prism.

Sample cross sections of the prisms 116, over a width w, are shown isFIGS. 2, 3 and 4. FIG. 5 is a graphical depiction of brightness as afunction of horizontal viewing angle for an optical substrate comprisinga surface characterized first 118 by a cross section of a right angled,straight-sided prism, second 122 by the curved sidewall or facet in FIG.3, and third 120 by the curved sidewall or facet in FIG. 4. As can beseen in FIG. 5, for a right angled, straight-sided prism 118 thebrightness shows significant side lobes 128, 130 at a horizontal viewangle of approximately ±50 degrees. These sidelobes are not seen ineither of the curved faceted prisms of FIGS. 3 and 4. However, there isa slight reduction in overall brightness for the curved prisms. As seenby comparing graph 122 with graph 120 in FIG. 5, for a refractive indexof approximately 1.6 in the optical substrate the steeper the curvatureof the side wall the greater the reduction in overall brightness. Also,as seen in FIG. 5, as the curvature of the facets increases away fromthe straight wall of a 90 degree prism, the wider is the central lobeand the lower is the central luminance and the sidelobes.

In a second embodiment, a relatively high index of refraction for theoptical substrate 110 in combination with a modified prism geometryyields an enhanced brightness. In particular, FIG. 8 displays a map ofthe central luminance in per cent of crossed optical substrates as afunction of the prism peak angle and the refractive index of thesubstrate, wherein a refractive index of 1.6 and a peak angle of 90degrees is taken to be 100 per cent. By increasing the peak angle to 100degrees and increasing the refractive index of the optical substrategenerally to greater than about 1.65 and in particular to betweenapproximately 1.7 and 1.8, the luminance is at least 102 per cent.

FIG. 9 shows a graphical depiction of the far field horizontal luminanceof crossed optical substrates as a function of horizontal viewing angle.In FIG. 9, a prior art luminance profile, based upon a refractive indexof 1.65 and a peak prism angle of 90 degrees is shown at 150. As can beseen in FIG. 9, the prior art shows sidelobes at 152. By increasing therefractive index of the substrates to about 1.75 and the peak prismangle to about 100 degrees, as seen at 154, the central portion of theluminance profile (e.g. ±30 degrees) displays a higher peak luminance(about 118) with essentially no sidelobes 156.

Similarly, FIG. 10 shows a graphical depiction of the far field verticalluminance of crossed optical substrates as a function of verticalviewing angle. In FIG. 10, a prior art luminance profile, based upon arefractive index of 1.65 and a peak prism angle of 90 degrees is shownat 158. As can be seen in FIG. 10, the prior art shows sidelobes at 160.By increasing the refractive index of the substrates to about 1.75 andthe peak prism angle to about 100 degrees, as seen at 162, the centralportion of the luminance profile (e.g. ±30 degrees) displays a higherpeak luminance (about 118) with suppressed sidelobes 164.

FIG. 11 shows a graphical depiction of the far field horizontalluminance of crossed optical substrates as a function of horizontalviewing angle. In FIG. 11, a prior art luminance profile, based upon arefractive index of 1.60 and a peak prism angle of 90 degrees is shownat 166. As can be seen in FIG. 11, the prior art shows sidelobes at 168.As further seen at 170 in FIG. 11, by increasing the peak angle from 90degrees to about 100 degrees while keeping the refractive index of thesubstrate at 1.60, the sidelobes 172 are reduced slightly while thecentral segment of the luminance is only slightly less. Still further,by increasing the refractive index of the substrates to about 1.75 andthe peak prism angle to about 100 degrees, as seen at 174, the centralportion of the luminance profile (e.g. ±30 degrees) displays a slightlyhigher peak luminance (about 105) with slightly lower sidelobes 176.

Similarly, FIG. 12 shows a graphical depiction of the far field verticalluminance of crossed optical substrates as a function of verticalviewing angle. In FIG. 12, a prior art luminance profile, based upon arefractive index of 1.60 and a peak prism angle of 90 degrees is shownat 178. As can be seen in FIG. 12, the prior art shows sidelobes at 180.As further seen at 182 in FIG. 12, by increasing the peak angle from 90degrees to about 100 degrees while keeping the refractive index of thesubstrate at 1.60, the sidelobes 184 are reduced while the centralsegment of the luminance is only slightly less. Still further, byincreasing the refractive index of the substrates to about 1.75 and thepeak prism angle to about 100 degrees, as seen at 186, the centralportion of the luminance profile (e.g. ±30 degrees) displays a higherpeak luminance (about 105) with slightly lower sidelobes 188.

Thus, it can be seen from FIGS. 8-12 that by increasing the refractiveindex of the substrate 110 and/or by increasing the peak angle, a, ofthe prism structures 116, an improvement is realized in an increase inthe on-axis luminance of the optical substrate 110 as well as areduction in the energy sidelobes of the horizontal and verticalluminance profile.

The optical substrate 110 may be formed from an optically transparentpolymer, an ultraviolet (UV) curable organic or inorganic material (orhybrid thereof). In such an optical substrate 110, an index ofrefraction of greater than about 1.65 is preferred.

Aside from the use of the optical substrates 110 described above inbacklight displays for brightness enhancement, the substrates can beused in a wide variety of other applications as well. Embodiments of thesubstrates 110 can be used in Fresnel lenses, hybrid glass/plasticlenses, optical disks, diffuser films, holographic substrates or incombination with conventional lenses, prisms or mirrors. Suchembodiments could be formed by modulating concentric circles or ellipseshaving fixed characteristics. The optical substrates can also be used insingle or multi-order reflective, transmissive or partiallytransmissive, devices, whether light absorbing or non light absorbing;prisms, holographic optical elements, or diffraction gratings. Thesubstrates can be used in other applications such as projectiondisplays, illuminated signs, and traffic signals. Another property ofthe invention is that the curved (or compound angle) facets increase theblurring of the light guide features. This is an advantage since itenhances the visual appearance of the display.

Any references to first, second, etc., or front and back, right andleft, top and bottom, upper and lower, and horizontal and vertical orany other phrase relating one variable or quantity to another are,unless noted otherwise, intended for the convenience of the descriptionof the invention, and are not intended to limit the present invention orits components to any one positional or spatial orientation. Alldimensions of the components in the attached Figures can vary with apotential design and the intended use of an embodiment without departingfrom the scope of the invention.

While the invention has been described with reference to severalembodiments thereof, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof Therefore, it is intended that the invention notbe limited to the particular embodiments disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. An optical substrate comprising: a surface comprising a prismstructure characterized by a cross section having a plurality of facetsdescribed by linear segments approximating a polynomial, whereinadjacent ones of the linear segments intersect at one side of acenterline of the prism, and wherein a peak angle of the prism structureis greater than 100 degrees and less than 105 degrees, and therefractive index of the substrate is between approximately 1.65 and 1.8.2. The optical substrate as set forth in claim 1, wherein the prismstructure comprises a plurality of prisms having a prescribed peakangle, α, a height, h, a length, 1, and a pitch, p.
 3. The opticalsubstrate as set forth in claim 2, wherein the plurality of prismsinclude at least a pseudorandom peak angle, α, height, h, length, 1, andpitch, p.
 4. The optical substrate as set forth in claim 1, wherein theprism structure is an ultraviolet curable organic or inorganic material.5. A backlight display device comprising: an optical source forgenerating light; a light guide for guiding the light therealongincluding a reflective device positioned along the light guide forreflecting the light out of the light guide; an optical substratereceptive of the light from the reflective device, the optical substratecomprising: a surface comprising a prism structure characterized by across section having a plurality of facets described by linear segmentsapproximating a polynomial, wherein adjacent ones of the linear segmentsintersect at one side of a centerline of the prism, and wherein a peakangle of the prism structure is greater than 100 degrees and less than105 degrees, and the refractive index of the substrate is betweenapproximately 1.65 and 1.8.
 6. The backlight display device as set forthin claim 5, wherein the prism structure comprises a plurality of prismshaving a prescribed peak angle, α, a height, h, a length, 1, and apitch, p.
 7. The backlight display device as set forth in claim 6,wherein the plurality of prisms include at least a pseudorandom peakangle, a, height, h, length, 1, and pitch, p.
 8. The backlight displaydevice as set forth in claim 5, wherein the prism structure is anultraviolet curable organic or inorganic material.
 9. A backlightdisplay device comprising: an optical source for generating light; alight guide for guiding the light therealong including a reflectivedevice positioned along the light guide for reflecting the light out ofthe light guide; an optical substrate receptive of the light from thereflective device, the optical substrate comprising: a surfacecomprising a prism structure characterized by a cross section having aplurality of facets including a first facet oriented at a first anglewith respect to the surface of the prism and a second facet oriented ata second angle with respect to the surface of the prism, wherein thefirst and second facets intersect at one side of a centerline of theprism and the first and second angles are different, and wherein a peakangle of the prism structure is greater than 100 degrees and less than105 degrees, and the refractive index of the substrate is betweenapproximately 1.65 and 1.8.
 10. The backlight display device as setforth in claim 9, wherein the prism structure comprises a plurality ofprisms having a prescribed peak angle, α, a height, h, a length, 1, anda pitch, p.
 11. The backlight display device as set forth in claim 10,wherein the plurality of prisms include at least a pseudorandom peakangle, a, height, h, length, 1, and pitch, p.
 12. The backlight displaydevice as set forth in claim 9, wherein the prism structure is anultraviolet curable organic or inorganic material.